Fractional distillation process for purifying triaryl phosphates



United States Patent 3,219547 FRACTIONAL DISTILLATION PROCESS FOR PURIFYING TRIARYL PHOSPHATES Edward N. Wheeler, Corpus Christi, Tex., assignor t Celanese Corporation of America, New York, N.Y., a corporation of Delaware Continnation of application Set. N0. 107,873, Feb. 27, 1961. This appiication Mar. 13, 1963, Ser. No. 268,853 2 Claims. (Cl. 202-40) This applicatin is a continuation of application Serial No. 107,873 filed February 27, 1961, now abandoned.

This invention relates to the purification of crude triaryl phosphates. More particularly, it relates to the removal of impurities from crncle triaryl phosphates.

Crude triaryl phosphates such as tricresyl phosphates contain a number of mpnrities which afect the quality of the phosphates. Several methods have been devised for purifying crude triaryl phosphates including the removal of the irnpurities by oxidation with potassium perm-anganate or by washing with caustic solutions. Many of these methods -produce a highly purified product.

F or many years, however, these skilled in the art have been seeking, without any substantial success, a method of purifying triaryl phosphates soiely -by distillation. Distillation methods were believed to be more desirable purification methods in that they would be less expensive, e.g., by eliminating the coet of oxidizing agent and other compositions used in purification and by reducing triaryl phosphates losses during puritication e.g. losses in wash- 1ng.

Of the rnany distillation processes developed, not one has snccessfully employed distiliation alone to remove the impurities. In each of these processes, it was found to be neeessary to ernp1oy additional steps to remove impurities which the distillation failed to completeiy remove.

Among the distillation processes used was one in which low boiling impnrities (impurities having a lower boiling point than the triaryl phosphate) were first distilled ofi, followed by the removal of the heart cut which contained the triaryl phosphate, thus leaving a residue containing high boiling impurities (impurities having a boiling pont higher than that of the triaryl phosphate).

The low and high boiling impurities, the removal of which this invention is concerned with, nclllde, as low boiling impurities isomers of phenol, present in the phenols rorn which the triaryl phosphates may be prodnced, thio phenol irnpurities usually a1so present in said phenols; where the triaryl phosphate has been prepared by the well known reaction of a chloride of an oxyacid of phosphorus and a phenol, then the lower boiling impurities further include monoaryl phosphorodichloridates diaryl phosphorodichloridates and a group of unidentified colored componnds which are believed to be oxidation products of the phenols.

The high boiling impurities include the catalyst, diary1 hydrogen phosphate, tetraaryl pyrophosphate and a group of unidentified colored compounds which are higher boil ing than tricresyl phosphate.

The heart cut then had to be washed with aqueous 3,2i9,547 Patented Nov. 23, 1965 caustic solution to remove impurities which were not removed during distillation. In another process, a heart cnt sirnilar to that descrbed above had to be treated with activated charcoal or other adsorbents to remove remaining impurities. In both the washing and the adsorbing steps, significant losses of triaryl phosphate occurred.

-It is an object of this invention to provide an mproved procedure for the -purification of triaryl phosphate.

It is another object of this invention to provide a process for the purification of triaryl .phosphate by distillation alone.

It is a further object of this invention to provide a continnous process for the purification of triaryl phosphates.

Other objects Wll be apparent from the following de tailed description and claims in which =all proportions are by weight unless otherwise stated.

I have now found that the aforementioned prior distillation procedures apparently failed to recognize the effects of auxiliary reactions inherent in these dstillations, which reactions made it -substantially impossible to achieve a high degree of purity in the -distillation products. I have discovered that under the temperature conditions at which the heart cuts containing the triaryl phosphates were distilled 0, the ten1per-atures at the base of the distillation apparatus where the high boiling impurities tended to concentrate were suficiently high to decompose such high boiling irrrpurites to produce low boiling impurities which distilled off with and contaminated the heart ent. I have noted that high boiling impurities particulariy tended to be deposited en the heating surfaces usually found in the base of distillation apparatns or in reboilers associated with the bases of fractional distillation apparatus which heating surfaces had temperatures higher than the ternperature of the reactants in the bases. Such deposits of high boiling impurities displayed even more marked decornposition. Further, in the described batch distillations of the prior art, the decomposition of high boiling impurties became more pronounced with the increasing concentration of the high boiling impurities as the reaction proceeded toward completion. Thus, after the distillation of the heart cut had zproceeded to a substantial extent, the heart cut removed contained substantal amounts of undesirable low boiling decomposition -products of high boiling impurities.

In accordance with one aspect of my invention I have found that substantially complete purification of triaryl phosphates may be achieved by a nevel continuons distillation in which low and preferably minimal concentrations of high boiling impurities are maintained. In this eontinuous distillation, a buildup of high boiling imp-urities such as that of the batch dstiliations of the prior art is avoided since fresh reactants are being continuously fed to the distillation zone and high boiling irnpnrities are being continuously removed from the distillation zone as a base stream.

In accordance with a further aspect of my invention, crude triaryl phosphate is continuously fed to a distillation zone from which a residue stream comprising high boiling impurities is continuously removed and triaryl phosphate is continuously taken overhead as a vapor which a1so contains low boiling impurities. It is preferable that the overhead vapor be substantially free of high boiling impurities.

In other Words, the temperatures under which the distillation is conducted should be below that at whch heavy ends will distill ofi overhead. Whether heavy ends are distilling over may be determined by removing a small sample of the overhead vapors and batch distilling from said sample all low boilng impurities. The residue comprising triaryl phosphate should have an acidity of less than 0.2 mg. KOH and preferably less than 0.1 mg. KOH per gram of sample unless undesirable amounts of high boilng impurities are also contained. Sample residues having an acidity of less than 0.2 mg. KOH per gram would indicate an overhead vapor having less than 0.1 wt. percent of diaryl hydrogen phosphate. If the residue of the test sample has an acidity greater than 0.4 mg. KOH per gram, the distillation process overhead temperatures should be lowered to a point where the tested sample yields residue having an acidity within the above limits. In embodiments, hereinafter described, in which part of the overhead vapors are returned to the distillation zone as reflux, the overhead temperature may be conveniently lowered by increasing the reflux.

After the high boilng impurities are removed, the low boilng impurities may be removed from the triaryl phosphate by distillation. These low boilng impurities whch are removed also contain low boilng decomposition products of high boilng impurities whch may have decomposed during the initial distillation for the separation of high boilin g impurities.

While it is preferred that substantially all of the high boilng impurities be removed prior to a final distillation in which low boilng impurities are separated from the triaryl phosphate, it is to be understood that the process of this inventon will stil] be eiective where a major portion of high boilng impurities are separated in the initial distillation. In such process, the final distillation is con ducted in a fractional distillation column to which the triaryl phosphate containing low boilng impurities and the remaining high boilng impurities are continuously fed. The low boilng impurities are continuously removed overhead, any remaining high boilng impurties are con tinuously removed as a residue base stream and triaryl phosphate is continuously removed as a sidestream from said column at a point having a temperature above the condensaton point of said low boilng impurities and below the vaporization point of high boilng impurities.

It is preferable that the final distillation be conducted at subatmospheric pressures. This is particularly true when the triaryl phosphates fed to the final zone contain some high boilng impurities. Subatmospheric pressures permit the distillation tobe carried out at lower temperatures, thereby maintaining decomposition of high boilng impurities at a minimum.

In another aspect of this inventon, the crude triaryl phosphate is continuously passed through a pluralty of initial fractional distillation zones to separate a major portion of high boilng impurities and to separate low boilng impurities from saidtriaryl phosphate. Then the triaryl phosphate is fed to a final distillation zone from whch low boilng impurities are removed overhead (from said final zone) any remaining high boilng impurities are removed in a base stream and triaryl phosphate is removed as a sidestream at a point having a temperature as hereinabove disclosed.

According to another aspect of this inventon the crude triaryl phosphate is advantageously subjected to a plurality of initial distillations including an initial distillation in which a major portion of the high boilng impurities are separated as heavy ends and an initial distillation in which a major portion of the low boilng impurities are separated as light ends. The initial distillation in which the low boilng impurities are separated is preferably a tillation without any reflux. The final distillation is preferably a fractional distillation.

The instant inventon has its greatest utility in the purification of tri(methyl substituted phenyl) phosphates such as tertiary phosphates of cresol or xylenol or the cresol or cresol-xylenol mixtures known as cresylic acid. However, it, is also suitable for purification of tertiary phosphates of other hydroxy aryl compounds. Specific instances of hydroxy aryl compounds are phenol; orthometa, and para-cresol; any of the six xylenols; any of the six trimethyl phenols; any of the three tetra-methyl phenols; penta-methyl phenol; substituted phenols such as the ethyl, isopropyl, n-propyl, nbutyl, sec.-butyl, tert.- butyl, amyl, etc. whch correspond to the series of methyl substituted phenols set forth above; phenols substituted with halogen, nitroand similar groups; etc. The triaryl phosphates may be produced in conventional marmer, as by reaction of an excess of the hydroxyaryl compound with phosphorus oxychloride in the presence of a catalyst for the reaction, e.g. magnesium chloride, stannic chloride, titanium tetrachloride and titanium esters. The process of this inventon may also be used to purify tricresyl phosphate prepared according to U.S. Patent No. 2,805,240 to W. Prahl. Further, this process may be employed to purify and reclaim used triaryl phosphate products.

The temperature, pressures, and residence times in the initial and final distillation zones will vary with the triaryl phosphate to be purified, In general, when the initial distillation is fractional distillation, the base pressure ranges preferably from about 15 mm. to 50 mm. Hg A and most preferably -from about 20 mm. to 30 mm., the overhead pressure ranges preferably from about 5 mm. to 35 mm. HgA and most preferably from 5 mm. te 10 mm. The temperature at which the initial distillation is conducted is below the temperature at whch high boilers distll overhead. This temperature varies with the composition of the crude mixture. As heretofore set forth samples of the overhead vapors may be tested or high boiler content and the temperature adjusted accordingly. For example in purifying tertiary phosphates of xylenol, it has been found that the overhead temperature of the initial zone is preferably in the range of trom about 260 to 285 and the base temperature in the range of trom about 300-320 C. When phosphates of phenol are being purified, the temperatures are preferably from 30 to 40 C. lower and when tricresyl phosphate is being purified the temperatures are from 15 to 20 C. lower than those given for Xylenol phosphates.

The rates of feed, removal of the overhead vapors and withdrawal of the residue are such that the residence time of the triaryl phosphate and the low boilers preferably range from about 6 to 30 minutes while the residue may have a residence time of over 1 hour and up to 7 hours or higher. At least part of this initial distillation may be conducted as a flash distillation that is a distillation without rectification under the same conditiens.

The final distillation is preferably conducted under the following conditions:

Base pressures trom about 15 mm. to 50 mm. HgA and most advantageously from about 20 mm. te 30 mm. Overhead pressure from about 5 to 35 mm. and most advantageously from about 5 to 10 mm. The tempeature of the final distillation will vary with the nature of the crude triaryl phasphate. For example, when triaryl phophates prepared from xylenol are used, the overhead temperature of the final distillation is preferably from to 230 C. and the base temperature is preferably from 300 to 320 C. When phosphates of phenol are being purified, the temperatures are preferably from 30 to 40 C. lower and when tricresyl phosphate is being purified, the temperatures are from 15 to 20 C. lower than those given for xylenol phosphates.

The residence times in the final distillation of the feed thereto is from about 6 to 30 minutes while the residue remaining of said zone may rernain for over one hour and up to seven hours or more.

Where the inltial dlstlllation and the final distlllation are combined in a contlnuous distillation, lt has been found to be advantageous to feed the initlal distillation zone, a refiux removed from said final distillation zone preferably the upper part of said final distillatlon zone.

The invention is illustrated further in the accompanying drawing in which FIG. 1 is a diagrammatic view of a preferred ernbodiment of this invention. FIG. 2 is a diagramrnatlc view of another embodiment of this inventlon. FIG. 3 is a diagrammatic view of stll another embodiment of thls invention.

As shown in FIG. 1, the liquid crude triaryl phosphate is continuously fed to initial fractional dlstillation column at point 11. Column 10 is equipped With reboiler 12. Resldue stream 13 containing high bolling impurities is continuously removed from column 10. Low bolling impurities and the bulk of the triaryl phosphate are continuously removed overhead from column 10 and contlnuously fed to final column 14 at polnt 15 which is preferably the tray below the top tray of the column. Column 14 is also equipped With a reboiler 16. Light ends are continuously removed overhead from column 14. A liquid sidestream 17 is continously withdrawn from column 14 and returned to column 10 as refiux. A residue stream 18 is continously withdrawn from column 14 and returned to column 10. Purified triaryl phosphate is withdrawn from final column 14 as vapor sidestream 19.

FIGURES 2 and 3 disclose continuous processes in which a major portion of both low bolling impurities and high bolling impurlties are eliminated before the final distillatlon zone.

With reference to FIGURE. 2, the crude trlaryl phosphate is contlnuously wthdrawn from flash distiller 20 and fed to fractional distillatlon column 22 equlpped With reboiler 23. Llght ends containing low bolling impurities are continuously removed overhead from column 22. Residue stream 24 is continuously removed from the base of column 22 and recycled to flash distller 20. Vapor sidestream 25 which contains the triaryl phosphate, is contlnuously removed from column 22 and fed to final distillation column 26 equipped With reboiler 27. Light ends are continuously removed overhead from column 26. Part of these light ends are returned to column 22 as reflux and part are returned to column 26 as reflux. A resldue stream 28 is continuously wlthdrawn from the base of column 26 and recycled to column 22. The purified trlaryl phosphate is wlthdrawn from column 26 as vapor sldestream 29.

With reference to FIG. 3, the crucle triaryl phosphate is continuously fed to flash dlstiller 30 equlpped With reboiler 31. The residue contalning heavy ends is continuously removed from distiller 30. The light ends are continuously wlthdrawn from distiller 30 and fed to fractional distillatlon column 32 equlpped With reboller 33. Light ends containing low bolling impurities are continuously removed overhead from column 32. Residue stream 34 is continuously removed from the bas e of column 32 and fed to final distillation column 35 equipped With reboiler 36. Light ends are continously removed overhead from column 35 and returned to column 32 as reflux. A residue stream 37 is continuously withdrawn from the base of column 35. The purified triaryl phosphate is withdrawn from column 35 as liquid sidestream 38, which is sufficiently high on the column to be above the point to which substantlally no high boiling i=mpurities may rise and suficlently low enough on the column to be at a temperature high enough to prevent the condensation of significant amounts of the low bolllng impurlties and low bolling decomposition products of high bolling impurlties so that low boilers and high boilers are substantially absent from the liquid sidestream in other than negliglble amounts.

The following example is given to lllustrate this invention further:

Example Crude liquid tricresyl phosphate made by the reactlon of cresyllc acid and phosphorus oxychloride in the presence of 0.2 wt. percent cresyl titanate having an APHA color of over 300, a permanganate demand of greater than 10 g. of KMIIO4 per hundred g. of sample and an acidity of 5 mg. KOH per g. was contlnuously fed at the rate of 600 parts per hour to the 2nd tray of an 8-tray initial dlstillation column 10 of an apparatus set-up as shown in FIGURE 1. The column 10 was operatcd at a base pressure of 28 mm. HgA, an overhead pressure of 19 mm. HgA, a base temperature of 308 C. and an overhead temperature of 270 C. Overhead vapors were contlnuously removed from the column at the rate of 600 parts per hour and a resldue stream was continuously removed from the base at the rate of 42 parts per hour. An additional 600 to 800 parts per hour of the overhead vapors were condensed and returned to column 10 as refiux. The crude tricresyl phosphate had an average residence time of 20 minutes in the column and reboiler while the resldue stream had a total average resldence time of 290 minutes (4.8 hours) primarily in the reboiler. The overhead vapors from column 10 were continuously fed to the 8th tray of a l0-tray distillation column 14. A 50-g. sample of the overhead vapors was condensed and then distilled at a pressure of 10 mm. HgA to an overhead tcmperature of 275 C. over a period of 50 minutes. About 20 g. of the sample was dlstilled overhead. A 20-g. sample of resldue had an acldity of 0.2 mg. KOI-I per g. It was tltrated to the phenolpthalein end polnt With 0.1 N caustic after refluxlng the sample for three minutes in an isopropyl alcohol solution contalning 10 wt. percent water. Column 14 was operated at a base pressure of 28 mm. HgA, an overhead pressure of 17 mm. HgA, a base temperature of 308 C., an overhead temperature of 210 C. and a reflux ratlon of 1 to 1. A resldue base stream of 42 parts per hour was continuously removed from column 14 and fed to the second tray of column 10. An overhead light ends stream was removed at a rate of 66 parts per hour. A vapor sidestream comprising the purlfied tricresyl phosphate was removed from the 2nd tray of column 14 at the rate of 490 parts per hour and condensed. The resulting tricresyl phosphate had an APHA color of 20, an acldlty of 0.03 mg. KOH per g. and an excellent permanganate time of more than several hours (half-hour minimum is requlred).

It is to be understood that the foregoing detailed description is glven merely by way of illustration and that many "ariations may be made therein without departing from the spirit of my inventlon.

Having descrlbed my nventlon what I desire to secure by Letters Patent is:

1. Process for purifying crude triaryl phosphate containing low-bolling and high-bolling mpurities which comprises feeding said crude to an initial fractional distillation column; continuously removlng -a residue stream contalnng substantlally all of the high bolling lmpurlties in said crude; continuously removlng as a vapor substantially all of the trlaryl phosphate together With the low bolling lmpurlties in said crude; feedlng said vapor to a final fractional dlstlllatlon column, operating at subatmospherlc pressure, near the top of said final column; recycling the base stream of said final column to said inltial column; refluxlng a liquid stream, taken from said final column near the top thereof to said initial column near the top thereof; continuously removlng low boilers overhead from said final column; and continuously removing the triaryl phosphate product as a vapor sidestream from said final column at a point below said reflux stream but above said recycle base stream.

2. Process for purifying crude triaryl phosphate containing low-boiling and high-boilng impurities which comprises feeding said crude to an initial fractional distillation column near the base of said initial column;

continuously removing a residue stream containng substantially all of the high bolling impurities in said crude;

contnuously removing overhead substantially all of the triaryl phosphate together with the low boiling impurities in said crude; feeding said overhead to a final fractional distillation column, operating at subatmospheric pressure, near the top of said final column; recycling the base stream of said final column to said initial column at a point above the point of crude feed; refluxing a liquid sidestream, taken from said final column near the top thereof but below the feed thereto, to said initial column near the top thereof; continuously removing low'boilers overhead from said fihal column; and continuously removing the triaryl phosphate product as a vapor sidestream from said final column -at a point below said liquid reflux sidestream but above said recycle base stream;

References Cited by the Examiner UNITED STATES PATENTS 1/1959 Bonstedt 2604 61.311

NORMAN YUDKOFF, Primary Examiner.

GEORGE D. MITCHELL, Examner. 

1. PROCESS FOR PURIFYING CRUDE TRIARYL PHOSPHATE CONTAINING LOW-BOILING AND HIGH-BOILING IMPURITIES WHICH COMPRISES FEEDING SAID CRUDE TO AN INITIAL FRACTIONAL DISTILLATION COLUMN; CONTINUOUSLY REMOVING A RESIDUE STREAM CONTAINING SUBSTANTIALLY ALL OF THE HIGH BOILING IMPURITIES IN SAID CRUDE; CONTINUOUSLY REMOVING AS A VAPOR SUBSTANTIALLY ALL OF THE TRIARYL PHOSPHATE TOGETHER WITH THE LOW BOILING IMPURITIES IN SAID CRUDE; FEEDING SAID VAPOR TO A FINAL FRACTIONAL DISTILLATION COLUMN, OPERATING AT SUBATMOSPHERIC PRESSURE, NEAR THE TOP OF SAID FINAL COLUMN; RECYCLING THE BASE STREAM OF SAID FINAL COLUMN TO SAD INITIAL COLUMN; REFLUXING A LIQUID STR%EAM, TAKEN FROM SAID FINAL COLUMN NEAR THE TOP THEREOF TO SAID INITIAL COLUMN NEAR THE TOP THEREOF; CONTINUOUSLY REMOVING LOW BOILERS OVERHEAD FROM SAID FINAL COLUMN; AND CONTINUOUSLY REMOVING THE TRIARYL PHOSPHATE PRODUCT AS A VAPOR SIDESTREAM FROM SIDE FINAL COLUMN AT A POINT BELOW SAID REFLUX STREAM BUT ABOVE SAID RECYCLE BASE STREAM. 